National Geographic News
An acoustic transponder used to sense movement of the ocean's floor.
An acoustic transponder that tracks earthquake movements is installed off Japan in an undated picture.

Photograph courtesy Japan Coast Guard via Science/AAAS

Richard A. Lovett

for National Geographic News

Published May 19, 2011

Japan's seabed shifted by as much as 79 feet (24 meters) in an east-west motion during the giant March 11 earthquake—the largest earthquake movement ever recorded, scientists say.

(See 20 unforgettable pictures of the aftermath of the Japan earthquake and tsunami.)

But that doesn't mean that it's the largest such shift ever to have been caused by an earthquake, cautioned Chris Goldfinger, director of the Active Tectonics and Seafloor Mapping Laboratory at Oregon State University.

The March earthquake was, however, the first time that scientists have directly measured such a slippage thousands of feet of underwater.

"Any magnitude 9 earthquake will have similar values," said Goldfinger, who was not part of the study team.

For instance, the 2004 Sumatra earthquake may have moved the seabed by as much as 100 feet (30 meters), he said by email.

(Related: "Japan Earthquake Shortened Days, Increased Earth's Wobble.")

GPS Technology Tracks Fault Movements

For several years, a team led by Mariko Sato of the Japan Coast Guard has been monitoring particular spots along the Japanese fault that produced the recent earthquake.

Scientists had placed transponders on the seabed. Using high-precision sonar techniques, the researchers could then record the transponders' locations from research vessels, whose own locations were carefully tracked by GPS satellites.

(Related: "Earthquake Fault Under Tokyo Closer Than Expected, Study Finds.")

This two-step technique is necessary, because GPS signals cannot reach the seabed, Sato said by email.

Shortly after the Japan earthquake occurred on March 11, the scientists returned to measure the changes.

"This is the first time a great subduction earthquake has been directly observed in the submarine part of the fault, which is where most of the action takes place," noted Oregon State's Goldfinger.

"We normally have to infer slips from onshore GPS," Goldfinger said. "Being able to measure it directly is very useful. It confirms the ability to model it from shore. It will help quite a lot in refining such models."

Understanding Future Tsunamis, Earthquakes

Overall, the more we learn about such earthquakes the better, Goldfinger added. Pre-2011 tectonic models, for example, did not predict a big earthquake around the site of the March epicenter near the east coast of Honshu island, Japan. (Read more about earthquake prediction.)

Furthermore, such research is useful in understanding tsunamis, research leader Sato said by email.

"It is important to continue monitoring seafloor movements in order to evaluate the risks of future earthquakes and tsunami."

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